A risk assessment methodology for evaluating the effectiveness of safeguards and determining safety instrumented system requirements

Many companies in the chemical and hydrocarbon process industries are increasing their efforts to determine the number and effectiveness of safeguards required to protect against potential accident scenarios at their facilities. One reason is to ensure that a company is operating within its risk acceptance criteria. But regardless of a company's motivation, a systematic methodology needs to be employed to assist in the analysis and implementation of safeguards. ISA standard ISA-S84.01-1996, Application of Safety Instrumented Systems for the Process Industries, provides a set of guidelines and requirements applicable to designing and managing safety instrumented systems (SISs); however, a methodology for analyzing the effectiveness of non-SIS safeguards in reducing risk and determining the need for an SIS is not included in the scope of the standard. Several critical steps must be performed to determine the effectiveness of safeguards. Because these steps involve making risk-based decisions, companies need to develop and implement systematic risk assessment methodologies to provide the information and basis for each decision. Applying these methodologies will help companies determine (1) if their risk acceptance criteria are being met and (2) when to implement safeguards to reduce the risk. This paper provides the following: An overview of issues to consider when establishing corporate or plant risk acceptance criteria A description of a semi-quantitative risk methodology that can be used as an effective, risk-based decision-making tool An approach for combining information from a process hazard analysis (PHA) with the described risk-based decision-making tool to determine the effectiveness of safeguards and the need for SIS implementation Using the method described in this paper will help companies better understand the adequacy of the safeguards installed in their facilities and will help identify the need to improve existing safeguards and/or implement additional safeguards, particularly SISs, to satisfy their risk criteria.